The present invention relates to a piezoelectric actuating device for controlling the flaps on the rotor blade of a helicopter, including a piezo element arrangement that contains at least one piezoelectric stack actuator, and further including a power transmission frame which is coupled to the piezo element arrangement and attachable to a rotor blade and which generates a force between an abutment and a driven element, which are provided on the power transmission frame, from a change in length of the piezo element arrangement upon excitation thereof, the force acting transversely to the direction of the centrifugal force of the rotor blade, provision being made for a first holding device via which the power transmission frame can be fixed to the rotor blade in the direction of the centrifugal force but which is flexible transversely to the direction of the centrifugal force and which allows the power transmission frame to move transversely to the direction of the centrifugal force relative to the rotor blade within a limited range; and provision being made for a second holding device which fixes the abutment, which is provided on the power transmission frame, transversely to the direction of the centrifugal force relative to the rotor blade but is flexible in the direction of the centrifugal force and which allows the abutment to move in the direction of the centrifugal force relative to the rotor blade within a limited range.
Piezoelectric actuating devices are increasingly used in many fields of application and can be used to advantage especially in aerospace due to their small size and their high power density. Because of their high power density, piezoelectric actuating devices are particularly suitable for controlling the flaps on the rotor blades of helicopters. In this context, however, it is a problem that the piezoelectric actuating device must be guaranteed to be securely supported and decoupled against the centrifugal force (of up to 1000 g) occurring at the rotor blade.
German Patent DE 196 46 676 C1 and German Patent Application DE 196 48 545 A1 describe piezoelectric stack actuators in the form of stacked individual piezoelectric elements as could be provided in a piezo element arrangement contained in a piezoelectric actuating device and used for driving the same.
Moreover, U.S. Pat. Nos. 4,808,874 and 4,952,835 describe piezoelectrically driven amplifier mechanisms which amplify a stroke movement without using levers or motors.
Furthermore, U.S. Pat. No. 5,907,211 describes piezoelectric actuator which has an X-shaped configuration and is operated by two piezoelectric elements. The actuator is arranged within a housing, the actuation being accomplished via a corresponding displacement plate. Moreover, the actuator is attached to the housing by flexible holding devices which are formed in one piece. The actuator is used, for example, for controlling the servoflap of a rotor blade of a helicopter.
An object of the present invention is to provide a piezoelectric actuating device for controlling the flaps on the rotor blade of a helicopter which features a high degree of performance reliability and operational safety and at the same time markedly reduces the risk of damage to the piezoelectric element.
The present invention provides a piezoelectric actuating device for controlling the flaps on the rotor blade of a helicopter, including a piezo element arrangement that contains at least one piezoelectric stack actuator, and further including a power transmission frame which is coupled to the piezo element arrangement and attachable to the rotor blade and which generates a force between an abutment and a driven element, which are provided on the power transmission frame, from a change in length of the piezo element arrangement upon excitation thereof, the force acting transversely to the direction of the centrifugal force of the rotor blade, provision being made for a first holding device which fixes the power transmission frame on the rotor blade in the direction of the centrifugal force but is flexible transversely to the direction of the centrifugal force and which allows the power transmission frame to move transversely to the direction of the centrifugal force relative to the rotor blade within a limited range and provision being made for a second holding device which fixes the abutment, which is provided on the power transmission frame, transversely to the direction of the centrifugal force relative to the rotor blade but is flexible in the direction of the centrifugal force and which allows the abutment to move in the direction of the centrifugal force relative to the rotor blade within a limited range. According to the present invention, the first holding device contains two arrangements of first flexurally soft straps extending essentially parallel to each other in the direction of the centrifugal force, and the second holding device contains two arrangements of second flexurally soft straps extending essentially parallel to each other transversely to the direction of the centrifugal force which will allow the flexurally soft straps of the first holding device and of the second holding device to perform a parallelogram movement.
The piezoelectric actuating device has an advantage of being reliably and securely fixed against the high centrifugal force occurring at the rotor blade of a helicopter without thereby impairing the performance reliability of the actuating device.
Advantageously, the first holding device includes first holding elements which extend longitudinally in the direction of the centrifugal force and which are attached to the rotor blade at one end and to the power transmission frame of the actuating device at the other end. These guarantee that the power transmission frame is reliably supported against the centrifugal force, the driven element and the abutment at the same time being allowed to move without restriction in the manner intended for this.
Preferably, the first holding elements are formed of straps which are flexurally soft transversely to the direction of the centrifugal force of the rotor blade. Such straps allow the power transmission frame to move without restriction transversely to the direction of the centrifugal force, the power transmission frame at the same time being reliably fixed in the direction of the centrifugal force. Moreover, the use of such flexurally soft straps allows the power transmission frame to be fixed in the third direction as well, i.e., in a direction perpendicular to the plane defined by the direction of the centrifugal force and the intended direction of motion of the abutment and the driven element.
Preferably, the first holding device contains two arrangements of first flexurally soft straps extending essentially parallel to each other in the direction of the centrifugal force. This permits a quasi parallelogram movement of the flexurally soft straps relative to the power transmission frame and relative to a centrifugal force fastener which is provided on the rotor blade and to which the first holding elements are fixed. In an embodiment, each of the two arrangements of first flexurally soft straps contains only one such strap so that the power transmission frame is held by two first straps running parallel to each other in the direction of the centrifugal force.
On the side of the power transmission frame, the first holding elements are advantageously attached to the abutment or in the vicinity thereof or to the driven element or in the vicinity thereof, respectively.
According to an embodiment of the piezoelectric actuating device according to the present invention, the second holding device includes second holding elements which extend transversely to the direction of the centrifugal force and are connected to the abutment at one end and, at the other end, to a fastening element which is used for attachment to the rotor blade. This allows the power transmission frame of the actuating device to move in a defined manner in the direction of the centrifugal force so that power transmission frame is decoupled from the centrifugal force to the largest extent possible with regard to the motion of the driven element and the abutment relative to each other.
Preferably, the second holding elements are also formed of flexurally soft straps which here, however, are arranged in such a manner that they are flexible in the direction of the centrifugal force of the rotor blade. This permits motion between the fastening element and the abutment in the intended direction in the direction of the centrifugal force, the power transmission frame, on the other hand, being fixed in a direction perpendicular to the plane defined by the direction of the centrifugal force and the working direction between the abutment and the driven element.
Preferably, the second holding device contains two arrangements of second flexurally soft straps extending essentially parallel to each other transversely to the direction of the centrifugal force. This, in turn, permits a parallelogram movement between the fastening element and the abutment; each of the two arrangements of second flexurally soft straps in the simples and preferred case containing only one such strap so that the second holding device is formed of two flexurally soft straps running essentially parallel to each other transversely to the direction of the centrifugal force.
Preferably, the piezo element arrangement of the actuating device contains at least one piezoelectric stack actuator extending longitudinally in its working direction, and the power transmission frame includes working yokes which are coupled to the respective ends of the piezo element arrangement as well as driven legs which connect the working yokes and extend approximately parallel to the longitudinal direction of the piezo element arrangement and which feature driven regions in the form of the abutment and the driven element in a central region between the working yokes, the driven legs together with the working yokes forming a flexible arrangement in terms of a displacement of the driven regions relative to each other in a direction perpendicular to the longitudinal direction of the piezo element arrangement, the flexible arrangement converting an expansion and contraction of the piezo element arrangement in its longitudinal direction to a movement of the abutment and the driven element relative to each other in a direction perpendicular to the longitudinal direction of the piezo element arrangement.
Preferably, the above mentioned embodiment is designed in such a manner that the piezo element arrangement is coupled with its ends to the center of each working yoke, respectively, and that each working yoke is provided with driven legs in pairs which oppose each other with respect to the center of the respective working yoke and which are each flexibly connected at one end to each side of a working yoke and which, at the other end, extend toward the driven regions in the form of the abutment and the driven element, respectively, which are located in the central region.
According to an embodiment of the piezoelectric actuating device according to the present invention, the driven legs are constituted by intrinsically rigid single legs which are hinged in the central region and at the ends that are associated with the working yokes.
According to a preferred refinement thereof, in each case two parallel, spaced apart driven legs are provided between each side of a working yoke and a driven region in the form of the abutment or the driven element, respectively, the driven legs performing a parallelogram-like movement during a movement of the abutment and the driven element. The special advantage of this is an increased stability of the power transmission frame against the arising high centrifugal forces.
Preferably, the first holding elements are provided at their end which is intended for attachment to the rotor blade with fastening loops which are integrally formed with the first holding elements,
According to an embodiment of the piezoelectric actuating device, provision is made for the power transmission frame, which includes the working yokes, the driven legs, and the driven regions in the form of the abutment and the driven element, to be manufactured in one piece together with the first holding elements and the second holding elements, including the fastening elements used for attachment to the rotor blade. This advantageously allows for inexpensive manufacture of a piezoelectric actuating device according to the present invention which features a high degree of reliability and operational safety.
According to an alternative embodiment, the driven legs can be formed of a thin flexible strap material instead of in the form of intrinsically rigid single legs, the strap material featuring a low flexural stiffness in the direction of its thickness and a high rigidity in the transverse direction.